Eric Hug

Effect of punching on electrical steels: Experimental and numerical coupled analysis

Punching electrical steels drastically alters their magnetic properties near the cutting edges, which should be accounted for when designing machines. A simple magneto-mechanical coupled finite element analysis, suitable for fully processed grades, is proposed in order to model the consequences of this local degradation of the material on the global behavior of a machine. The model is used to study the effective section of the teeth of a punched stator; calculated results are compared with experimental data and a good agreement is found.

Magnetic behaviour versus tensile deformation mechanisms in a non-oriented Fe–(3 wt.%)Si steel

Abstract This study presents an extended in situ magnetic characterisation of a non-oriented (NO) Fe–(3 wt.%)Si steel. An appropriate experimental device was created and magnetic measurements were performed under uniaxial tensile stresses approaching and exceeding the macroscopic elastic limit σ e and in the corresponding unloaded states. Both Barkhausen noise and B–H hysteresis loops were measured. The sensitivity to stress was found to be qualitatively similar to that of polycrystalline iron. The different stages of the tensile deformation (perfectly elastic stage, microplastic yielding stage, the two strain-hardening stages) were clearly identified by the magnetic parameters. In the plastic strain domain, the coercive field H c and the inverse of the initial relative permeability 1/ μ r i linearly increase, while the maximal relative permeability μ r max and the Barkhausen noise peak height BN max linearly decrease with the applied stress σ . The remnant induction B r keeps a low and constant value. Furthermore, a linear dependence of 1/ μ r i , H c , μ r max and BN max on the kinematic hardening X was found. By using measurements on prestrained specimens under reloaded elastic stresses, an accurate identification of the effect of dislocations acting as pinning sites and of the magnetoelastic effect of long-range internal stresses was proposed.

Effect of internal stresses on the magnetic properties of non-oriented Fe–3wt.% Si and (Fe,Co)–2wt.% V alloys

Abstract The magnetic properties of soft magnetic materials are very sensitive to mechanical strengthening and especially to plastic straining. In a first approximation, the degradation of the magnetisation with the plastic strain level is supposed to be roughly isotropic. However, even after uniaxial strengthening, the multiaxial residual stresses induce directional fluctuations of these degradations. An experimental device has been created in order to study the effect of such internal stresses on magnetic behaviour. Experimental results are given for non-oriented Fe–3wt.% Si and (Fe,Co)–2wt.% V thin sheets. An attenuated effect of the strengthening is observed in the direction perpendicular to the applied stress whatever the direction of the latter inside the sheet plane. This phenomenon is discussed in terms of metallurgical defects (dislocation structures, grain boundaries and antiphase boundaries), internal stresses and magnetocrystalline anisotropy.

Magnetic characterisation of elastically and plastically tensile strained non-oriented Fe–3.2%Si steel

Abstract The effects of uniaxial tensile stresses approaching and exceeding the macroscopic elastic limit on the magnetic properties of a non-oriented Fe–3.2%Si steel sheet are analysed. The evolutions of the maximal relative permeability, coercivity and Barkhausen noise energy in the loaded and corresponding unloaded states are reported, attesting the influence of the long-range internal stresses on the magnetic degradation during mechanical hardening.

Some aspects of the magnetomechanical coupling in the strengthening of nonoriented and grain-oriented 3% SiFe alloys

An investigation has been carried out on the effect of plastic strains on the magnetic properties of grain-oriented and nonoriented 3% by weight SiFe alloys. A drastic degradation of these properties with increasing deformations is observed for magnetic field amplitudes ranging between 0 and 2000 A/m. Empirical relationships between plastic strain and magnetic characteristics are obtained. Materials exhibit a Luders strain state under tensile loading in a low plastic deformation range. Meanwhile, the classical Ramberg-Osgood law is verified. The observation of the dislocation features at various plastic strain levels shows three typical configurations: hexagonal cells in the Luders strain state, small tangles and isolated screw dislocations at medium values of strain, and finally high-density tangles at higher deformations. In the same way, the densities of main and secondary magnetic domains follow an evolution in three stages with increasing strains. It is shown that the transverse domain patterns take place to counterbalance the increase of magnetoelastic energy due to the strengthening. The evolution of the coercivity and the initial relative permeability with the strains can be explained using potential model theories for the grain-oriented alloy in the range [2-8]%. Domain wall bowing theories could successfully be applied to both alloys at the ultimate stage of the strengthening. The relationship between the coercivity and the strengthening displays two linear stages for both 3% SiFe alloys instead of the three stages ordinarily reported in the case of polycrystalline high-purity iron.

Influence of the plastic anisotropy on the magnetic properties of a nonoriented 3% silicon iron

Modifications of the magnetic properties of a nonoriented 3% SiFe alloy with plastic strains are reported. The samples have been tested along rolling and transverse directions lying in the sheet plane by means of a suitable experimental system of measurements using a 50 Hz sinusoidal magnetic field. The magnetic properties markedly deteriorate when measurements are carried out in the direction of the applied stress. This phenomenon is less important perpendicular to the applied stress. Quasistatic experiments (f=0.1 Hz) have been accomplished to separate the total losses into hysteresis and dynamic components. Hysteresis losses strongly increase and dynamic losses sensitively decrease with the plastic deformation whatever the direction in sheet plane. Compression‐tension fatigue tests carried out have shown that the material displays a strong kinematic strengthening behavior representative of internal and long range stresses in the strained sample. The degradation of the magnetic properties of nonoriented 3% SiFe alloys with plastic strains is the result of two effects: one due to dislocations and another due to internal stresses. The latter display along the direction of magnetic measurements the same effects as an outside elastic stress.

Structural analysis and thermoelectric properties of mechanically alloyed colusites

We describe here a new, easy and scalable route for synthesising colusites by using mechanical-alloying and reactive spark plasma sintering, together with the thermoelectric behaviour of zinc-substituted derivatives, Cu26−xZnxV2Sn6S32 (0 ≤ x ≤ 2). X-ray diffraction analysis coupled with transmission electron microscopy evidences the high crystallinity of the as-synthesized samples. In the pristine compound, an intrinsic exsolution phenomenon leads to the formation of two distinguishable colusite phases. Additional neutron powder diffraction results support the substitution of Zn in the tetrahedral Cu sites and 119Sn Mossbauer spectroscopy analyses prove the presence of only Sn4+ in colusite irrespective of the Zn content (x ≤ 2). The mechanical properties denote a wide homogeneity of the samples despite a significant impact of the exsolution on the microstructure. The Zn for Cu substitution provides electron doping, decreasing the holes concentration. High temperature thermoelectric properties in the p-type series Cu26−xZnxV2Sn6S32 are reported. The highest power factor of 0.92 mW m−1 K−2 at 700 K is found for x = 1, with a corresponding ZT value of 0.4.

Size Effects in Thin Face-Centered Cubic Metals for Different Complex Forming Loadings

Influence of the size effects on the mechanical behavior of face-centered cubic metals was studied for complex loadings close to microforming ones. The effect of a reduction in thickness (t) over grain size (d) ratio on the mechanical behavior for high-purity nickel and copper is investigated for three different loadings by tensile and Nakazima tests (plane strain conditions and balanced biaxial expansion). Experimental results highlight a strong degradation of the mechanical properties of Cu and Ni when the t/d ratio is reduced below a critical value, independently of the strain path. However, this effect occurs if the equivalent plastic strain is larger than a critical level which is strain path dependent and related to the stress triaxiality. The current study reveals that plastic anisotropy is also affected by size effects. An excellent correlation is obtained between the t/d ratio and the thickness reduction, through the mean normal plastic anisotropy parameter which is widely used to estimate sheet formability. A size effect map based on forming limit diagrams is proposed to depict the optimal conditions of microforming.

Growth and texture of spark plasma sintered Al2O3 ceramics: A combined analysis of X-rays and electron back scatter diffraction

Textured alumina ceramics were obtained by Spark Plasma Sintering of undoped commercial α-Al2O3 powders. Various parameters (density, grain growth, grain size distribution) of the alumina ceramics, sintered at two typical temperatures 1400 °C and 1700 °C, are investigated. Quantitative textural and structural analysis, carried out using a combination of Electron Back Scattering Diffraction and X-ray diffraction, are represented in the form of mapping and pole figures. The mechanical properties of these textured alumina ceramics include high elastic modulus and hardness values with high anisotropic nature, opening the door for a large range of applications.

Damage mechanisms evolution of ductile cast irons under thermomechanical loadings

In this work, tensile and compression tests were carried out on ferritic cast iron samples in order to understand the influence of the stress state for temperatures until 800°C. Results show an asymmetric behaviour of ductile cast iron (DCI). The damage created during hardening was evaluated using local microstructural quantifications. Several damage differences are highlighted at room temperature. Therefore, when temperature increases, differences between damage mechanisms under tension and under compression are amplified. A relation between nodules deformation experimentally determined at different stress-strain stages and numerical simulation is proposed. The numerical predictions are in good agreement with the distribution of the aspect ratio (the ratio between major and minor axis of nodules) obtained experimentally. It was noted that nodules deformation can represent the total material strain at room temperature. In addition, it was shown that graphite nodules in cast iron cannot always be assimilated to voids during the strain.